Stackable extension



FIG. 1 is a right rear perspective view of a stackable extension showing my design.

FIG. 2 is a left front perspective view thereof.

FIG. 3 is a front view thereof.

FIG. 4 is a rear view thereof.

FIG. 5 is a right side view thereof, the left side view being a mirror image thereof.

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof.

The broken lines shown herein are for illustrative purposes only and form no part of the claimed design. 

The ornamental design for a stackable extension, as shown and described. 